1. Field of the Invention
The present invention relates generally to cardiac pacemakers, and more particularly to an exercise-responsive implantable cardiac pacemaker in which the stimulation rate is adaptively regulated in part according to the central venous blood temperature of the patient.
2. Relevant Background
In situations where the natural pacemaker or pacing system of a patient's heart is disturbed because of age, disease or injury, it is customary to employ artificial pacing with an implanted cardiac pacemaker.
In the prior German patent No. 34 19 439 of Eckhard Alt, and corresponding application for U.S. patent Ser. No. 747,111, copending with and assigned to the same assignee as is the present application, a pacing rate-adaptive circuit for a cardiac pacemaker is disclosed in which the stimulation rate is controlled according to the central venous blood temperature of the patient and to the variation of blood temperature commensurate with one or the other of two different types of characteristic curves. These curves functionally describe the relationship between heart rate and central venous blood temperature, one of the curves being representative of a resting state of the patient and referred to as the basic curve (reference or resting function), and other curves being representative of exercise states of the patient and referred to as exercise curves or functions. The exercise curves are superimposed on the basic curve, each of the former having a higher slope than the latter and each intersecting the basic curve at a different reference temperature point. The stimulation rate of the cardiac pacemaker is controlled according to either the resting function or an exercise function depending on both (1) the the time rate of change of blood temperature relative to a threshold value preselected as the criterion for distinguishing between states of rest and exercise, and (2) the instantaneous blood temperature.
In the apparatus described in the aforementioned copending Alt application, the stimulation rate of the pacemaker is controllably varied in the same direction as the variation of blood temperature. Thus, a rise of blood temperature produces an increase in heart rate, and a drop of blood temperature results in a decrease in heart rate. When the patient is in an initial state of rest, which may include sleeping, reclining, sitting or standing without significant accompanying physical activity, the pacing rate is variably regulated under the control of the resting function based on then-current blood temperature level and the determination that the time rate of change of blood temperature lies below the preselected threshold value. When the patient commences physical activity sufficient to cause his central venous blood temperature to rise at a rate exceeding the threshold value, that condition is detected and results in a shift of pacing rate control from the reference function to that exercise function appropriate to the instantaneous level of his blood temperature.
In U.S. Pat. No. 4,436,092 to Cook et al., issued Mar. 13, 1984, a cardiac pacemaker is disclosed in which the stimulation rate is varied according to a single mathematical function derived from an observed relationship between blood temperature and heart rate for a normally functioning heart under stress. According to Cook, et al., various constants in the exercise algorithm representing this relationship were derived principally from experimental data on test dogs. The algorithm is used in conjunction with the output voltage wave of a temperature sensor to control the output repetition frequency of the pacemaker's pulse generator. A similar description is also given by Csapo et al. in the German patent OS No. 26 09 365, and in their paper entitled "Autoregulation of Pacemaker Rate by Blood Temperature" presented at the VIII World Congress of Cardiology in Tokyo, Japan in September 1978, as well as by a publication in Verh. Dtsch. Ges. Kreislaufforsch. 44:1978, page 152, entitled "Frequenzsteuerung von Schrittmachern durch Bluttemperatur." In the Cook et al. U.S. Pat. No. 4,436,092, the controlled variation of stimulation rate, and thus of the patient's heart rate, is also in the same direction as the change of the patient's central venous blood temperature, albeit the variation is limited to a single mathematical function.
Subsequent Cook et al. U.S. Pat. No. 4,543,954, issued Oct. 1, 1985, describes a pair of embodiments of an exercise responsive stimulation rate controlled cardiac pacemaker. In one embodiment, the stimulation rate is set at either of two discrete levels, one of which is a resting rate and the other an exercise rate. In another embodiment, three discrete settings are available, one of which is intermediate the other two. In both, an abrupt "either-or" type of shift between rates occurs in each direction based on whether or not the time derivative of blood temperature is respectively more positive or more negative than preselected set points, in contrast to normal physiological changes of heart rate. Again, the change in stimulation rate is in a direction consistent with that of the change of blood temperature.
In fact, however, the rise of blood temperature does not occur at the instant the physical exertion begins; rather, it is time delayed relative to the latter. At commencement of exercise, the central venous blood temperature typically dips, that is, drops slightly, owing to the rapid circulation of initially relatively cooler blood from the extremities to the heart. The heart temperature gradient of blood in the human body decreases in the direction from the heart to the periphery of the body. At the onset of physical exercise, there is increased circulation not only in the working musculature but also in the extremities. The lower temperature of the extremities initially tends to cool the blood flowing through them, for at least a brief interval of time.
Indeed, in individuals with below normal cardiac output, maintenance of sufficient blood circulation through the life-sustaining organs, such as the liver, kidneys and brain, is assured at the expense of lower circulation of the extremities and skin. Thus, patients with abnormal heart function experience lower temperature in the extremities than do persons with normally functioning hearts. This tends to compound the duration and extent of the blood temperature dip at the commencement of physical exercise in cardiac patients.
The temperature drop continues until the heat generated by the working muscles balances out the colder blood inflow from the extremities, thereby causing the central venous blood temperature to cease dropping and, with continued exercise, to rise. The extent and duration of this initial temperature dip depends on the functional state of the heart and circulatory system, the skin temperature and level of initial amount of exertion by the individual. In general, for persons with normal circulation in a state of rest, and who are clothed and not cool, the blood temperature drop may last from about twenty to about thirty seconds. As noted above, the duration of the drop may be considerably greater for persons with below normal blood circulation.
The initial exercise-prompted blood temperature drop has been reported in the literature; by Bundin in the Scan. J. Clin. Lab. Invest., 35, 1975, page 542, and with more recent results, by E. Alt et al. in the Journal Herzschrittmacher, Munich, 5 (1985), No. 2, page 66 et seq.
Accordingly, exercise-responsive cardiac pacemakers in which the stimulation rate is based on blood temperature typically will undergo a time delayed response to exercise, or even a drop of pacing rate at onset of exercise. This would appear to be the case, for example, with the regulation apparatus disclosed in the aforementioned Cook et al. patents. The exercise responsive pacemaker disclosed in the aforementioned copending Alt application has a stimulation rate controlled by the basic (resting) curve of heart rate as a function of central venous blood temperature, in a manner that will lead to only a slight decrease in the stimulation rate with the initial temperature drop according to regulation along the basic curve. On the other hand, it has been observed that for healthy persons the initiation of physical exertion is accompanied by an elevation of the heart rate, despite the dip in blood temperature. Therefore, it would be desirable to provide an exercise-responsive cardiac pacemaker which is controlled in a manner that the initial dip in blood temperature is detected, to trigger an adjustment of the stimulation rate corresponding to the heart rate of a healthy person with good cardiovascular circulation undergoing similar physical exercise at that blood temperature level.
It is a principal object of the present invention to provide an improved implantable cardiac pacemaker in which there is rapid recognition of the onset of exercise to produce a short term variation of pacing rate closely paralleling the heart rate of a normal healthy person under the same conditions.
Another object of the present invention is to develop criteria for use in detecting the initial drop of blood temperature as a reliable indicator of the onset of exercise to trigger an increased stimulation rate.
A further important object of the invention is to provide means to distinguish the initial drop of blood temperature attributable to the commencement of exercise from other situations in which the patient's central venous blood temperature may experience a short term drop, such as occurs following the completion of physical exercise.